1. Field of the Invention
The present invention relates to precursor compositions that are useful for the deposition of conductive electronic features. The precursor compositions can advantageously have a low conversion temperature to enable low-temperature treatment of the precursors to form conductive features on a variety of substrates. The precursor compositions have a relatively high viscosity and can be deposited onto a substrate using methods such as thick-film deposition or syringe dispensing.
2. Description of Related Art
The electronics, display and energy industries rely on the formation of coatings and patterns of conductive materials to form circuits on organic and inorganic substrates. The primary methods for generating these patterns are screen printing for features larger than about 100 μm and thin film and etching methods for features having a feature size not greater than about 100 μm. Other subtractive methods to attain fine feature sizes include the use of photo-patternable pastes and laser trimming.
Another consideration with respect to patterning of conductors is cost. Non-vacuum, additive methods generally entail lower costs than vacuum and subtractive approaches. Some of these additive methods utilize high viscosity flowable liquids referred to as pastes. Screen-printing, for example, utilizes pastes with viscosities of thousands of centipoises. At the other extreme, low viscosity compositions can be deposited by methods such as ink jet printing.
The ideal starting material and printing technique for the fabrication of conductive features and coatings using high viscosity pastes would combine a number of attributes. The final conductor would have a high conductivity, preferably close to that of bulk silver or copper. When the conductor included a metal, the metal would be dense and pure. In cases where thermally sensitive substrates were required, the processing temperature would be low enough to allow the formation of conductors without damage to the organic substrates. The conductor would have high resistance to electromigration, solder leaching and oxidation. The methods and compositions would be capable of forming alloys with useful properties.
In the case of conductors derived from pastes containing combinations of molecular precursors and powders, a variety of criteria would be met. The deposited material would not spread or slump once deposited, thereby maintaining line width and thickness. The deposited paste would have a high yield of conductor. The deposit would adhere strongly to the substrate and would be compatible with the substrate and other materials in contact with the conductor. The deposit would be flexible and mechanically strong. In cases where ease of processing is most critical, UV curable compositions would be desirable.
Existing thick film conductor compositions cannot provide this combination of features. Polymer thick film compositions typically include a precursor to a polymer (e.g., an epoxy) and a conductive filler (e.g., silver) for conductive compositions. These pastes provide processing temperatures close to 100° C., but offer poor reliability and performance. Thin film techniques offer high reliability and high performance, but have a high cost and provide limited materials and geometries. Paste compositions including conductor particles that must be sintered require firing temperatures of 600° C. or higher, limiting their application to glass or ceramic substrates. Thus, there is a need for paste compositions that provide a combination of high performance, low processing temperature, and low cost.
U.S. Pat. Nos. 6,036,889 and 5,882,722 by Kydd disclose conductor precursor compositions that contain metallic particles, a precursor (metal organic composition compounds) and a vehicle that provide conductors at low temperatures on organic substrates.
For example, U.S. Pat. No. 6,036,889 discloses compositions including metal flakes having a thickness of about 1 μm.
U.S. Pat. No. 6,197,366 by Takamatsu discloses methods using inorganometallic compounds to obtain formulations that convert to dense solid metals at low temperatures.
Attempts have also been made to produce metal-containing compositions at low temperatures by using a composition containing a polymer and a precursor to a metal. See, for example, U.S. Pat. No. 6,019,926, by Southward et al. However, the deposits were chosen for optical properties and were either not conductive or poorly conductive.
U.S. Pat. Nos. 5,846,615 and 5,894,038, both by Sharma et al., discuss precursors to Au and Pd that have low reaction temperatures thereby conceptually enabling processing at low temperatures to form metals.
U.S. Pat. No. 5,176,744 by Muller discloses the use of Cu-formate precursor compositions for the direct laser writing of copper metal. The compositions include a crystallization inhibitor to prevent crystallization of Cu-formate during drying.
There exists a need for high viscosity precursor compositions for the fabrication of conductive features for use in electronics, displays, and other applications. Further, there is a need for precursor compositions that have low processing temperatures to allow deposition onto organic substrates and subsequent heat treatement, while still providing a feature with adequate electrical and mechanical properties. It would also be advantageous if the compositions could offer enhanced resolution control.